Running title: Radiogenesis of Low-mature Natural Gas

Radiogenesis of Low-mature Natural Gas in the Turpan-Hami Basin, NW

China WANG Wenqing

1, LIU Chiyang

1,*, ZHANG Dongdong

1, LIANG Hao

2

1State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, Shannxi,

China 2PetroChina Turpan-Hami Oilfield Company, Xinjiang 839009, China

Abstract: The genesis of the Jurassic low-mature natural gas in the Turpan-Hami Basin has attracted scientists’ attention for

a long time, and it is known that radiogenesis may have played a role. However, little has been done on the uranium-rich

background over the whole basin. Based on plentiful logging and geological data of the Jurassic strata in the Turpan-Hami

Basin, this paper studied the features and controlling factors of high-gamma rocks distribution. The results show that

70%-100% of the high-gamma rocks correspond to mudstones in the prodelta subfacies rather than those in semideep-deep

lacustrine subfacies rich in mudstones. Therefore, we propose that the distribution of high-gamma rocks is mainly controlled

by sedimentary facies rather than by lithology. Further analysis of the gamma spectrometry logging data shows that high

gamma values are more strongly correlated with U contents compared to Th or K contents. By comparing the U and Th

contents of felsic rocks in periphery provenances, we find that the Jueluotage Mountain and Harlik Mountain were the

dominant uranium sources for the Jurassic Turpan-Hami Basin. The radiolysis due to high level uranium in the prodelta

subfacies can make the low-mature source rocks generate H2 and CH4, thus contributing to the production of low-mature

natural gas in the Turpan-Hami Basin.

Key words: high-gamma rocks, prodelta subfacies, uranium, radiolysis, low-mature natural gas, Turpan-Hami Basin

E-mail: nwulcy@163.com; lcy@nwu.edu.cn (Chiyang Liu)

1 Introduction

Extending from the Caspian Sea in the west to the Songliao Basin in the east, the Central-East Asian Multi-Energy Minerals Metallogenetic Domain (C-EAMD) encompasses more than 6,000 km and includes dozens of large-scale oilfields, gas fields, coalfields and sandstone-type uranium districts, many of which coexist within the same basin (Li, 2000; Gan et al., 2007; Liu et al., 2007, 2016; Cai et al., 2015; Liu et al., 2017; Zhang et al., 2018). The Turpan-Hami Basin, which is located in the southeastern part of the C-EAMD (Wu and Zhao, 1997; Wang et al., 1998; Zhang, 1998; Liu et al., 2007), is a typical basin in which multiple energy resources coexist. Although this basin is relatively small in size, its Jurassic strata bear abundant oil, gas, coal and uranium resources (Wang et al., 1998; Xu et al., 2008, 2009; Shen et al., 2010; Chen et al., 2001; Tang et al., 2007; Wu et al., 2009; Wang et al., 2011), including one of the biggest sandstone-type uranium deposits (Shihongtan) (Wu et al., 2009) and a typical low-mature natural gas province in China (Xu et al. 2008; Wang et al., 1998; Zhang et al., 2005; Shen et al., 2010; Wang et al., 2008; Zhao, 2013).

According to statistics, total production of low-mature gas reached to 632.15×1011

m3

and the production intensity is up to 100×10

8m

3/km

2, which indicates low-mature gas in this basin has considerable potential for

exploration (Lu et al., 2009). The gas and source rock correlation shows that the natural gas has a very good correlation with the Jurassic source rocks, especially with the middle and lower Jurassic coal-bearing source rocks (Wang et al., 1998; Wang et al., 2008; Shen et al., 2010; Zhao, 2013). The Ro measurements of the Jurassic source rocks all over the Turpan-Hami Basin indicate that most of source rocks in Jurassic are low-mature (Cheng, 1994; Wang et al., 2008; Lu et al., 2009; Xu et al., 2008, 2009; Zhao, 2013). Numerous previous studies have indicated that the resin, suberinite, sulfur in eight ring forms compounds and biodegradation in source rocks contributed to the generation of abundant low-mature gas (Wang et al., 1998; Wang et al., 2008; Xu et al., 2009; Lu et al., 2009). These compounds can lower the activation energy required for hydrocarbon generation, which is favorable for the natural gas production in low-maturity (Zhong and Wang, 1995).

It was proposed that water radiolysis led to the generation of hydrogen (Rabiei et al., 2017; Richard, 2017; Chi et al., 2018) and low-mature hydrocarbon could be generated from low-mature source rocks through radiolysis (Liu et al., 1999). In this regard, it is worth noting that the largest uranium deposits (Shihongtan) in the Turpan-Hami Basin is developed in the Jurassic layers. Besides, a previous basin-wide geochemical surface survey in the Turpan-Hami Basin, which analyzed 30 elements of fine-grained soil from the clay-rich horizon at a density of approximately one site per 100 km

2 (Wang et al., 2007, 2011), showed that in addition to the

Shihongtan uranium deposit, the entire basin is rich in uranium. However, seldom research has been conducted on the potential relationship between radioactivity and generation of low-mature gas in this basin.

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This article has been accepted for publication and undergone full peer review but has not been

through the copyediting, typesetting, pagination and proofreading process, which may lead to

differences between this version and the Version of Record. Please cite this article as doi:

10.1111/1755-6724.13852.

In this paper, in order to further examine whether the radiogenesis plays a role in low-mature gas generation, natural gamma-ray logging data from 162 wells throughout the Turpan-Hami Basin, gamma spectrometry logging data from 10 wells with U, Th and K contents at every measuring point, and diagrams of the sedimentary facies of the Jurassic strata were collected. The characteristics of high-gamma rocks in the Jurassic layers and the factors affecting the distribution of high-gamma rocks are studied. Furthermore, in consideration of the radiolysis experiments conducted by others, the geological implications of the distribution characteristics of high-gamma rocks are discussed.

2 Geological Setting

The Turpan-Hami Basin, which is close to the Santanghu Basin to the Northeast, the Junggar Basin to the

north and the Tarim Basin to the south, is one of the major petroliferous basins within the Xinjiang Uygur Autonomous Region in northwestern China. Tectonically, the W–E–trending Turpan-Hami Basin is located in the southeastern part of the Central Asian Orogenic Belt (CAOB) (Fig. 1a) and is bounded by Tianshan Orogenic Belt (the Bogda Mountain to the north, Harlik Mountain to the northeast and Jueluotage Mountain to the south) (Fig.1b; Ge et al., 1997; Shao et al., 1999; Chen et al., 2001; Pirajno et al., 2008; Wu et al., 2009). Structurally, the Turpan-Hami Basin consists of three tectonic units: the Turfan Depression in the west, Liaodun Uplift in the midsection, and Hami Depression in the east. Turfan Depression contains the Tuokexun Sag, Taibei Sag and Tainan Sag (Fig. 1b; Chen et al. 2001). From the Carboniferous to the Permian, the Turpan-Hami Basin underwent three sedimentary environments: (1) marine facies of the extensional stage; (2) marine–lacustrine transitional facies of the tectonic transition stage; and (3) lacustrine facies of the compression and inversion stage (Jiang et al., 2015). In the Mesozoic, the Turpan-Hami Basin consisted entirely of lacustrine facies (Fig. 1c; Shao et al., 1999; Chen et al., 2001; Min et al., 2005).

The Jurassic strata, which are the focus of this study, comprise the Badaowan Formation (J1b), Sangonghe formation (J1s), Xishanyao Formation (J2x), Sanjianfang Formation (J2s), Qiketai Formation (J2q) and Qigu Formation (J3q). Among these Jurassic formations, the coal-measure source rocks of the Badaowan formation and the Xishanyao formation along with the Jurassic lacustrine mudstone of the Qiketai formation in Jurassic are the main source rocks of low-mature natural gas in the Turpan-Hami Basin (Fig. 1c).

Fig. 1(a), Simplified geological map of the Turpan-Hami Basin (modified after Xie et al., 2016) (China basemap after China

National Bureau of Surveying and Mapping Geographical Information); (b), A brief introduction to the structural units of the

Turpan-Hami Basin (modified after Greene, 2005; Ni et al., 2015); (c), Stratigraphy column of the Jurassic strata in

Turpan-Hami Basin (modified after Chen et al., 2001; Gong et al., 2016)

3 Study Methods

Natural gamma-ray logging data for 162 wells, gamma spectrometry logging data for 10 wells and basic

geological data were compiled in the Turpan-Hami Basin. The gamma spectrometry logging data include a gamma value, the U, Th and K contents per 0.125 m interval. The average gamma values of mud-stone in most areas of the Turpan-Hami Basin are 50–70 API, while those of coal and sandstone are slightly lower; gamma values higher than 100 API are considered as high-gamma values. First, for the natural gamma-ray logging data,

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each well was divided into different formations: J1 (J1b and J1s), J2x, J2s, J2q and J3q. Second, in each formation, gamma values exceeding 100 API were sifted for analysis. Each gamma value represents 0.125 m rock layers; thus, the thickness of high-gamma value rocks in each formation was determined by multiplying the amount of the sorted high-gamma values by 0.125 m. Third, by comparing the thickness distribution of high-gamma rocks with sedimentary facies laterally and comparing the gamma values with lithology vertically in each well, the sedimentary facies and rocks types with high-gamma values were determined. Based on the above steps, this paper investigates the distribution features of high-gamma rocks and the controlling factors.

4 Spatial-temporal Distribution Characteristics of High-gamma Rocks in Each Formation in Jurassic and Correlation Coefficients between U, Th, K Contents and Gamma Values

The map of sedimentary facies in each formation is overlaid by the isopach map of the thickness of high-gamma rocks (Fig. 2). Among the 162 wells, J1, J2x, J2s, J2q and J3q have high gamma values in 43, 72, 77, 50 and 35 wells, respectively. Since J1, the lake deepened firstly, and in J2x, the lake area was the largest. The lake was then reduced, resulting in the deposition of a large amount of sandstone in J2s. In J2q, the lake deepened again, making J2q one of the major source rocks of the Turpan-Hami Basin. Subsequently, the lake was again reduced (Fig. 1c and Fig. 2). Lateral comparison shows that the regions with high-gamma values are mostly distributed in delta facies (Fig. 2). In the following section, we will discuss the vertical distribution characteristics of high-gamma rocks in each formation.

4.1 Distribution characteristics of high-gamma rocks in J1

Two and four high-gamma areas developed in the Tuokexun Sag and Taibei Sag, respectively, and all high-gamma areas are distributed in delta facies (Fig. 2a). Six wells were selected to study the vertical distribution characteristics of high-gamma rocks [Fig. 3(1)a]: Tuocan1 (253m, the thickness of high-gamma values), Wusu2 (308 m), Ai2 (241 m), Yan3 (80 m) and Huo8 (101 m). There were four normal sedimentary cycles in J1, and the water body deepened in J1 overall. From bottom to top, gravel sandstone, fine sandstone, muddy siltstone and mudstone were developed, and a large amount of coal developed in the period of dereliction. Statistically, 64%–83% of rocks with high gamma values are mudstone, while 14%–23% are muddy siltstone and siltstone. The gamma values of a small amount of sandy conglomerate and coal are greater than 100 API. 4.2 Distribution characteristics of high-gamma rocks in J2x

Three high-gamma areas developed in both the Tuokexun Sag and the Taibei Sag. Most high-gamma areas are distributed in delta facies (Fig. 2b). Wusu2 (197m), Huo8 (108m), Ge8 (150m) and Mi1 (88m) were selected to study the vertical distribution characteristics of high-gamma rocks in J2x [Fig. 3(1)b]. Several sedimentary inverse cycles developed in J2x, and the water body shrank from bottom to top overall. In the period of dereliction, thick coarse sandstone and a small amount of coal developed. In the period of transgression, thick mudstone with muddy siltstone and packsand laminas deposited. Based on large amounts of data, 75%–90% of rocks with high-gamma values correspond to mudstone, while 6%–15% correspond to muddy siltstone and siltstone in J2x. A small amount of sandy conglomerate and coal have gamma values greater than 100 API [Fig. 3(1)b].

4.3 Distribution characteristics of high-gamma rocks in J2s

One and four high-gamma areas developed in the Tuokexun Sag and the TaibeiSag, respectively. Most high-gamma areas are distributed in delta facies (Fig. 2c). Wusu2 (58 m), Yan3 (26 m), Shenbei4 (87 m), Liansha2 (140 m) and Ge8 (32 m) were selected to study the vertical distribution characteristics of high-gamma rocks in J2s [Fig. 3(2)c]. Several normal cycles developed in J2s. In each cycle, coarse sandstone with coal laminas developed on the bottom, while thick gray mudstone with siltstone and muddy siltstone laminas developed on the top. The rocks with high-gamma values almost all correspond to mudstone.

4.4 Distribution characteristics of high-gamma rocks in J2q

Five high-gamma areas developed in the Taibei Sag, and all high-gamma areas correspond to delta facies (Fig. 2d). Shenbei4 (42 m), Shenna2 (43 m), Huo8 (26 m) and Liansha2 (71 m) were selected to study the vertical distribution characteristics of high-gamma rocks in J2q [Fig. 3(2)d]. Thick packsand, siltstone and muddy siltstone with mudstone laminas developed on the bottom, while thick mudstone with silty mudstone and a small amount of siltstone developed on top. This indicates that the lake water deepened overall in J2q, making J2q one of the significant source rocks in the Turpan-Hami Basin. Generally, 70%–95% of the rocks with high-gamma values correspond to mudstone, while 15%–20% correspond to muddy siltstone. A small amount of sandy conglomerate and coal possess gamma values greater than 100 API. 4.5 Distribution characteristics of high-gamma rocks in J3q

Two high-gamma areas developed in the Taibei Sag; both are distributed in shallow lake facies (Fig. 2e). Shenbei4 (101 m), Shennan2 (70 m) and Liansha2 (445 m) were selected to study the vertical distribution characteristics of high-gamma rocks in J3q [Fig. 3(2)e]. The range of the lake is the smallest compared to those

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developed in other stages. The largely developed thick purple mudstone with purple silty mudstone and a small amount of gray silty mudstone laminas represent an onshore sedimentary or shallow-water environment. The gamma values of more than 445m purple mudstone in Liansha2 well exceed 100 API.

4.6 Correlation coefficients between gamma values and radioactive elements

Generally, three radioactive elements (U, Th and K) contribute to the bulk of gamma values of sediments (Tan et al., 2007; Pehlivanli et al., 2014). Table 1 shows the U, Th (ppm), K (%) and corresponding high natural gamma values (> 120API) sifted from the gamma spectrometer logging data of Shennan 2 drill hole. The correlation coefficients between U, Th, K and gamma values are 0.97, -0.19 and 0.27, respectively (Fig. 4). We can see that the natural gamma values have the best correlation relationship with U contents.

Fig. 2 Spatial-temporal correlation between high-gamma rocks distribution and sedimentary facies of J1 (a), J2x (b), J2s (c),

J2q (d) and J3q (e). The sedimentary maps are after Zhao (2013). Red lines are the isopach of high natural gamma (> 100 API)

rocks.

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Fig. 3(1) Vertical distribution of gamma values in J1 (a) and J2x (b)

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Fig. 3(2) Vertical distribution of gamma values in J2s (c), J2q (d) and J3q (e)

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Table 1 List of U (ppm), Th (ppm) and K (%) contents and corresponding gamma values (API) (>120) from the gamma spectrometer logging data of Shennan 2

Depth/m 3013.375 3013.5 3013.625 3013.75 3016.375 3016.5 3016.625 3016.75 3016.875 3017.125 3017.25 3017.375 3017.5 3017.625 3017.75 3017.875 3018

GR/API 125.323 128.613 127.374 122.523 134.421 156.073 182.314 210.228 236.668 273.456 282.115 285.543 284.832 280.744 274.036 263.975 249.425

U/ppm 6.329 6.508 6.492 6.248 6.05 9.006 12.82 16.648 19.913 23.853 24.89 25.461 25.546 25.138 24.412 23.281 21.463

Th/ ppm 5.442 5.786 5.628 4.99 7.852 7.359 6.627 6.262 6.342 6.277 5.707 5.35 5.561 6.182 6.57 6.433 5.895

K/% 1.906 1.848 1.822 1.864 1.943 1.787 1.673 1.649 1.726 2.164 2.393 2.488 2.405 2.197 2.006 1.911 1.922

Depth/m 3018.125 3018.25 3018.375 3018.5 3018.625 3018.75 3018.875 3019 3019.125 3019.25 3029 3029.125 3029.25 3124.25 3124.375 3124.5 3124.625

GR/API 228.958 204.696 179.606 157.26 141.331 131.51 126.663 124.68 123.235 121.033 123.107 125.114 123.164 124.522 132.785 137.225 137.131

U/ ppm 18.583 14.984 11.279 8.195 6.405 5.785 6.054 6.752 7.364 7.636 6.063 5.456 4.524 5.783 6.135 6.44 6.637

Th/ ppm 5.405 5.334 5.64 6.094 6.573 6.968 7.093 6.825 6.216 5.588 6.872 7.799 8.535 7.08 8.079 8.306 7.9

K/% 2.015 2.134 2.245 2.313 2.286 2.185 2.07 1.98 1.896 1.776 2.028 2.096 2.21 2.216 2.251 2.276 2.277

Depth/m 3124.75 3124.875 3125 3125.125 3125.25 3125.375 3125.5 3125.625 3125.75 3125.875 3126 3126.125 3126.25 3126.375 3126.5 3126.625 3126.75

GR/API 134.944 132.836 132.3 133.817 136.174 138.242 139.174 138.019 134.846 130.236 125.405 122.31 121.788 123.549 126.154 127.263 125.917

U/ ppm 6.704 6.655 6.549 6.449 6.368 6.318 6.348 6.513 6.754 7.016 7.258 7.373 7.243 6.817 6.178 5.538 5.076

Th/ ppm 7.529 7.479 7.643 7.838 8.089 8.42 8.652 8.367 7.497 6.278 5.179 4.772 5.113 6.076 7.25 7.926 7.819

K/% 2.266 2.274 2.338 2.461 2.583 2.643 2.61 2.489 2.335 2.17 1.996 1.852 1.802 1.875 2.042 2.22 2.339

Depth/m 3126.875 3129.875 3130 3130.125 3130.25 3130.375 3130.5 3130.625 3130.75 3130.875 3131 3131.125 3131.25 3131.375 3131.5 3131.625 3131.75

GR/API 122.494 122.757 125.982 130.251 135.95 142.83 149.835 154.918 157.145 156.595 154.389 152.281 151.427 152.223 154.728 159.109 164.562

U/ ppm 4.804 9.573 10.005 10.16 10.262 10.435 10.715 11.053 11.308 11.431 11.529 11.77 12.106 12.4 12.545 12.514 12.411

Th/ ppm 7.101 2.541 3.09 3.724 4.127 4.153 3.825 3.358 3.049 2.97 2.994 2.911 2.676 2.415 2.326 2.581 3.019

K/% 2.388 1.332 1.28 1.337 1.501 1.746 2.002 2.145 2.137 2.008 1.836 1.729 1.746 1.88 2.064 2.21 2.306

Depth/m 3131.875 3132 3132.125 3132.25 3132.375 3132.5 3132.625 3132.75 3132.875 3133 3133.125 3133.25 3133.375 3133.5 3133.625 3133.75 3133.875

GR/API 170.184 175.058 178.572 181.97 187.609 197.22 209.078 217.562 218.797 212.572 202.569 194.474 190.025 187.918 185.926 183.009 178.87

U/ ppm 12.315 12.253 12.235 12.442 13.146 14.405 15.763 16.427 15.985 14.632 13.097 12.05 11.543 11.342 11.316 11.428 11.558

Th/ ppm 3.457 3.831 4.171 4.473 4.647 4.595 4.32 4.041 3.94 4.081 4.387 4.671 4.867 4.984 5.016 4.958 4.819

K/% 2.39 2.5 2.637 2.738 2.763 2.734 2.712 2.727 2.754 2.755 2.72 2.691 2.692 2.722 2.728 2.68 2.591

Depth/m 3134 3134.125 3134.25 3134.375 3134.5 3134.625 3134.75 3134.875 3135 3135.125 3135.25 3135.375 3135.5 3135.625 3135.75 3135.875 3136

GR/API 173.33 166.757 161.182 159.573 164.9 178.837 195.74 208.987 212.989 204.673 189.998 174.888 164.345 162.12 165.968 172.368 176.797

U/ ppm 11.469 11.017 10.57 10.618 11.483 13.036 14.491 15.281 15.115 13.935 12.323 10.793 9.777 9.601 10.017 10.636 10.959

Th/ ppm 4.659 4.51 4.301 3.995 3.69 3.687 4.067 4.661 5.21 5.48 5.509 5.416 5.272 5.056 4.807 4.624 4.607

K/% 2.495 2.437 2.424 2.432 2.43 2.425 2.456 2.543 2.671 2.797 2.862 2.83 2.714 2.581 2.514 2.55 2.664

Depth/m 3136.125 3136.25 3136.375 3136.5 3136.625 3136.75 3136.875 3167.25 3167.375 3167.5 3167.625 3207 3207.125 3207.25 3207.375

GR/API 174.378 165.697 153.46 141.117 131.685 125.337 121.583 120.387 124.056 124.228 120.625 122.384 123.601 122.293 120.254

U/ ppm 10.523 9.48 8.151 6.948 6.324 6.28 6.665 5.196 5.52 5.913 6.101 4.289 4.419 4.673 5.045

Th/ ppm 4.869 5.351 5.892 6.245 6.142 5.661 4.985 4.67 4.752 4.372 3.783 10.793 9.771 7.881 6.015

K/% 2.755 2.746 2.65 2.511 2.35 2.182 2.023 2.508 2.454 2.307 2.123 1.897 2.054 2.2 2.289

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Fig. 4 Correlations of gamma values with U, Th and K contents of Jurassic layers in the Shennan2 well

In order to analyze the correlation coefficients between gamma values and U, Th and K contents systematically, correlation coefficients of U, Th and K with gamma values (>100 API and >120 API) of 10 wells are calculated (Table 2) and the corresponding data are listed in attachment 1.

The results (Table 2) show that, when gamma values are over 100 API, the correlation coefficients between gamma values and U contents (average 0.59) are higher than that of Th contens (average 0.18) or K contents (average 0.03). However, there are exceptions; the correlations of gamma values with Th contents for Yan3, Aishi1, Shenbei4 wells are stronger than that of gamma values with U contents. When the gamma values are over 120 API, with the exception of in Mi1, the correlation coefficients between gamma values and U contents are stronger or comparable with those for gamma values above 100 API. However, the correlations between gamma values and Th or K contents are weakened to different degrees.

Table 2 Correlation coefficients between gamma values (more than 100 and 120API) and U, Th and K contents of

Jurassic layers in 10 wells

Wells Gamma values more than 100API Gamma values more than 120API

U Th K U Th K

Wusu2 0.67 0.13 0.01 0.90 0.11 -0.33

Yan3 0.24 0.37 -0.11 0.14 0.09 0.13

Huo8 0.89 -0.34 -0.15 0.86 -0.13 0.02

Aishi1 0.37 0.50 0.21 0.59 0.17 -0.12

Ge8 0.53 0.20 -0.06 0.80 -0.26 -0.69

Shenbei4 0.50 0.66 0.46 0.47 0.32 0.04

Guo4 0.93 0.04 -0.31 0.98 0.34 -0.66

Liansha2 0.62 0.09 0.18 0.84 -0.39 -0.34

Shennan2 0.91 -0.05 0.09 0.97 -0.19 0.27

Mi1 0.24 0.17 -0.03 -0.33 0.74 0.13

5 Discussion

5.1 Source of U and its enrichment progress

In Jurassic, the south Jueluotage Mountain and the northeast Harlik Mountain are the dominant provenance regions for the Turfan and Hami Depressions (Hendrix et al., 1992; Shao et al., 1999). In a previous study, Wang et al. (2018a) have confirmed that the felsic rocks in denuded areas of Yin’e, Beishan group, Santanghu, and the Juggar basins are rich in uranium and in long geological history; the uranium-rich felsic rocks have provided large amounts of uranium into adjacent sedimentary basins. The felsic rocks in Jueluotage Mountain and Harlik Mountain are verified to have provided lots of uranium for Turpan-Hami Basin using the same method as that in Wang et al. (2018b).

In nature, uranium is primarily quadrivalent and hexavalent (Li et al., 2010). The Quadrivalent uranium is not stable in the supergene zone. Superficial water with abundant dissociated oxygen will oxidize UO2 in denuded regions or relatively shallow uranium source layers into UO2

2+ and migrate (Li, 2000; El-Bayes and

Arnous, 2015). Before migrating into a reductive environment, uranium will be deposited in the redox transitional zone due to reduction and absorption (Mu, 1999; Li, 2000; Gan et al., 2007; Guo et al., 2018). As the onshore part of the delta facies, the delta plain is characterized with the paragenesis of sandy sediments and coal. The delta front below the horizontal plane is the dominant part of delta facies sandstone, while the prodelta consisting of dark clay and silt clay with a small amount of packsand develops the thickest sediments in the delta facies (Zhu, 2008). As the underwater part of delta facies, prodelta subfacies contain abundant organic matter and generally belong to the redox transitional zone. The above superposition analyses of thickness distribution of high-gamma rocks with sedimentary facies laterally and gamma values with lithology of single well vertically indicate that most high-gamma areas correspond to the prodelta subfacies, while a few high-gamma areas correspond to the delta front and delta plain. On the other hand, although most high-gamma values correspond to mudstone, the thickness of high-gamma rocks in the center of the lake with the thickest mudstone is small. This suggests that compared to lithology, sedimentary facies have a more dominant control over the distribution of high-gamma rocks.

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Based on these analyses, the uranium enrichment process is inferred to have taken place as follows. With the mechanical denudation of the Jueluotage and Harlik Mountains in the Jurassic, the clastic rocks rich in uranium migrated from both mountains into the Turpan-Hami Basin, resulting in synsedimentary pre-enrichment of uranium. Besides, since Jurassic, the quadrivalent uranium in the provenance rocks and also in the Jurassic layers in oxidizing zone have been leached during the chemical weathering and migrated with oxygenated water in hexavalent uranium. Finally, in the redox transition zones (prodelta subfacies), the hexavalent uranium was converted into quadrivalent uranium and deposited. All the above make the prodelta subfacies in Jurassic layers rich in uranium. Paradoxically, the gamma values of Jurassic layers of Mi1 well in the Hami Depression exhibit a weak relationship with the uranium contents (Table 2). Considering that the Jurassic layers in the Hami Depression are exposed to the surface for a long time, it is possible that the uranium in rocks might have migrated away. By the way, thick purple layers, which are not favorable for the deposition of uranium, in Liansha2 well in J3q Formation, are rich in uranium. It is probably that the original sedimentary environment was reductive; then, the rocks were oxidized for a long time period with the shrinking of the lake. Nevertheless, because the sedimentation area is also the discharge area, the uranium in J3q did not migrate. Inversely, oxygenated water carried the uranium in the provenance area to this discharge area, causing the oxidized discharge area to be rich in uranium. 5.2 Geological implications of uranium-rich rock distribution in source rocks

It is generally known that ionizing radiation in water leads to a number of ionic and excited states that further decompose or recombine to give radicals and molecular species including e

-aq, H·, OH·, H2O2, O2, H2 and

HO2· (Pastina and LaVerne, 2001). According to previous studies, reactions 1-7 shown below are responsible for the formation of molecular hydrogen in water radiolysis (Pastina et al., 1999, 2001; Le et al., 2005; Cecal et al., 2008; Fourdrin et al., 2013; Chupin et al., 2017). Firstly, ionizing radiation of water leads to the production of electrons (equation 1) and then they can be hydrated by water molecules to form hydrated electrons, e

-aq

(equation 2). On the one hand, these hydrated electrons are known to lead to H2 production (equation 4). On the other hand, the hydrated electrons can combine with H

+ to form H radicals (equation 3), and H2 can also be

created by reaction 5. In addition, equations 6 and 7 also contribute to the generation of H2. It is worth noting that the products in equation 1 are incomplete because of the complexity of the reaction, so equation 1 is not balanced. All of the reactions are in the radiated environment. H2O→ H· + OH· + H

+ + OH

- + e

- (1)

e- + H2O → e

-aq (2)

e-aq + H

+ → H· (3)

e-aq + e

-aq + 2H2O → H2 + 2OH

- (4)

e-aq + H· + H2O → H2 + OH

- (5)

H· + H2O → H2 + OH· (6) H· + H· → H2 (7)

The above data show that the high-gamma rocks mainly distribute in the prodelta subfacies which are important parts of source rocks and the high-gamma values have the highest correlation with U content comparing with that of the Th and K contents. That is to say, in Turpan-Hami Basin, high-gamma values are mainly contributed from uranium content. Taking Shennan2 well as an example, the thickness of the layers with gamma values higher than 120 API (the gamma spectrometry logging data shown in attachment 1) is about 15 m, and the average U content is 11 ppm. According to calculations, source rocks with 11 ppm

U has being exposed

to radiation fields as great as 0.66×10-7

Gy/h (Rong, 2002) since the sediment of Jurassic source rocks (about 170 Ma). The total absorbed dose in this case could be as high as 1×10

5 Gy (1 Gy = 1 J/kg). The experiment

conducted by Chupin et al. (2017) demonstrates that 2%-9% radiated energy was used for generating hydrogen efficiently because of different water ratio in rocks. It is known that 484 kJ energy is needed to produce 1 mol H2 from water. 2%-9% radiated energy in Turpan-Hami Basin can generate about 0.004 mol-0.019 mol H2 per kilogram source rocks. Besides, in the experiment of Chupin et al. (2017), gamma rays are used. However, in geological environment uranium atom releases not only γ rays, but also α rays and β rays (Lu, 2001) and according to the investigation of Wang et al. (2018a), given the same radiation energy, the ratio (G(H2))alpha

particles/G(H2)γ-rays of free water is about 3.4. That is, more H2 must have been generated in source rocks in the Turpan-Hami Basin than that calculated by gamma rays only.

Hydrocarbon formation is a well-known process of adding H and removing O, N, S and other heteroatoms (Hunt, 1996). There is a strong possibility that large amounts of exogenous H2 from water radiolysis could be involved in hydrocarbon generation in actual geological environments (Seewald, 2003). The exogenous H2 can react with unsaturated, long-chain hydrocarbons and sulfur or nitrogen compounds in certain temperature and pressure to produce saturated and short-chain hydrocarbons, which not only can optimize crude oil but also increase the quantity of oil and gas (equations 8-10; Zhang and Gao, 1996; Matar and Hatch, 2001; Mapiour et al., 2010; Dincer, 2012; Zhang et al., 2012). R’CH=CH2 + H2 (catalyst) → R’CH2CH3 (8) RCH2CH2CH2R’ + H2 → RCH2CH3 + R’CH3 (9) R-SH + H2 → RH + H2S (10) CO2 + 3H2 (with microbial processes) → CH4 + H2O (11)

Recently, our unpublished experiments have shown that methane can be produced directly via the radiolysis

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of liquid organic matters (e.g. decane) and large amounts of CO2 can be generated from the radiolysis of pitch and kerogen. It is known that CO2 can react with H2 to generate CH4 during microbial processes (equation 11; Liu and Zhang, 2009). Furthermore, in addition to generating radiated products, the residual radiated energy is all absorbed by source rocks, thus contributing to improving the maturity of source rocks (Tan et al., 2007).

Based on the above consideration, uranium-rich coal-measure source rocks in Jurassic are considered to have contributed to the low-mature gas (CH4) generation in the Turpan-Hami Basin.

6 Conclusions

(1) Among rocks with high gamma values, 70%-100% correspond to gray, dark or gray-green mudstone or silty mudstone in the prodelta subfacies. However, the thickness of high-gamma rocks in the center of the lake which is rich in mudstone, is small. That is to say, sedimentary facies have a more dominant control over the distribution of high-gamma rocks than lithology.

(2) Generally, high-gamma values are more strongly correlated with U contents compared to Th and K contents. The correlation between gamma values and U contents becomes stronger as gamma value increases.

(3) In the Jurassic, as the dominant provenances of the Turpan-Hami Basin, the felsic rocks in the Jueluotage Mountain and the Haerlike Mountain and also the relatively shallow uranium source layers provided most of the uranium for Turpan-Hami Basin.

(4) Large amounts of uranium in the source rocks could radiate water, hydrocarbon and solid organic matter to generate exogenous H2, CH4 and CO2, which contribute to the generation of low-mature natural gas in the Turpan-Hami Basin.

Acknowledgements

We thank the PetroChina Turpan-Hami Oilfield Company for kindly providing subsurface datasets, and for

the permission to publish the results of this study. This work was supported by the National Natural Science Foundation of China [grant numbers 41330315, 41402093].

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About the first author Wang Wenqing is currently a Ph.D. candidate majoring in Mineral prospecting and exploration of geology at Northwest University (China). She received her B.Sc. from Northwest University (China). Her research interests concern geochemistry and petroleum geology.

About the corresponding author Liu Chiyang is currently a professor of geology at Northwest University (China). He received his B.Sc. and M.Sc. degrees from Northwest University (China). His academic research career spans almost 40 years in the fields of basin analysis and petroleum geology.

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Figure captions

Fig. 1(a), Simplified geological map of the Turpan-Hami Basin (modified after Xie et al., 2016) (China basemap after China

National Bureau of Surveying and Mapping Geographical Information); (b), A brief introduction to the structural units of the

Turpan-Hami Basin (modified after Greene, 2005; Ni et al., 2015); (c), Stratigraphy column of the Jurassic strata in

Turpan-Hami Basin (modified after Chen et al., 2001; Gong et al., 2016)

Fig. 2 Spatial-temporal correlation between high-gamma rocks distribution and sedimentary facies of J1 (a), J2x (b), J2s (c),

J2q (d) and J3q (e). The sedimentary maps are after Zhao (2013). Red lines are the isopach of high natural gamma (> 100 API)

rocks.

Fig. 3(1) Vertical distribution of gamma values in J1 (a) and J2x (b)

Fig. 3(2) Vertical distribution of gamma values in J2s (c), J2q (d) and J3q (e)

Fig. 4 Correlations of gamma values with U, Th and K contents of Jurassic layers in the Shennan2 well

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Table caption Table 1 List of U (ppm), Th (ppm), K (%) contents and corresponding gamma values (API) (>120) from the gamma

spectrometer logging data of Shennan 2

Table 2 Correlation coefficients between gamma values (more than 100 and 120API) and U, Th and K contents of Jurassic

layers in 10 wells

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